One of antireflection surfaces which is applicable to an optical storage medium is described on pages 993 to 1009 of "Optica Acta, 1982, Vol. 29, No. 7". The antireflection surface is called "Moth Eye" and comprises a very fine array of protuberances thereon. Each of the protuberances is round in its plane, and the diameter thereof is approximately 1/3 .mu.m as described later. In the antireflection surface, the reflection factor is as low as several percent, although the theory and manufacture thereof are not described here in detail.
There has been proposed an optical storage medium utilizing such an antireflection surface which comprises a plastic substrate on which a very fine array of protuberances are formed, and a metal layer covering the protuberances. The diameter of each of the protuberances is much smaller than that of a recording pit which will be described later.
In the optical storage medium, a laser beam is radiated in accordance with information stored in a memory such that the laser beam is passed through the plastic substrate to reach the metal layer. As a result, the metal layer absorbs the laser beam and is heated up to a predetermined temperature so that a portion of the plastic substrate which is in contact with the metal layer thus heated is softened. The softened plastic substrate becomes relatively flat due to the surface tension thereof which tends to decrease the surface area of the portion. As a result, a flat portion into which an image signal is written is formed as illustrated later.
In the optical storage medium into which information is written as described above, the reflection factor of the flat portion is increased from several percent to several tens of percent. Therefore, information stored therein can be read out thereof due to the difference in the reflection factors between the flat portion and non-flat portion of the protuberances.
There has been proposed another optical storage medium on pages 662 to 664 of "Applied Physics Letters, Vol. 40, No. 8" published on Apr. 15, 1982. The optical storage medium comprises a glass substrate, and a Ge layer deposited on the glass substrate. A random array of submicron columns are formed on the surface of the Ge layer by reactive ion etching. The formation of the submicron columns results in an optical antireflection surface due to the graded refractive index produced by the textured surface.
In writing image data into the optical storage medium, the submicron columns are melted in accordance with the information carried by a laser beam. As a result, reflective spots are formed locally due to the collapse of the columnar structure, although the details thereof are not described here.
In the both former and latter optical storage medium, however, there are the following disadvantages. The first disadvantage is that it is rather difficult to provide a very fine array of the protuberances called "Moth Eye" on the plastic substrate which are smaller in the diameter than a wavelength of light, and to provide a random array of the columns of crossectional dimensions less than 100 nm on the surface of a Ge layer. The second disadvantage is that S/N ratio of information which is read out of the optical storage medium is not as high as expected because the reading signal of the information is based on the difference between the intensities of light which are reflected from the surface including the protuberances and from the flat portions. In other words, light passing through the plastic substrate is changed in its intensity due to a stain, crack etc. on the back surface thereof even if incident light is maintained at a constant level. As a result, the S/N ratio mentioned above is decreased especially in an optical storage medium such as an optical card which is liable to receive such a stain or crack on the back surface thereof during a practical use thereby reducing its reliability.